Because the bonding pair of electrons is localised, on average, closer to the nuclei of the bonded atoms.
The covalent bond is an electrostatic attraction between the positively-charged nuclei of the bonded atoms and the negatively-charged binding pair of electrons in their #sigma-#orbital. Electrostatic attractions are short-range forces and their strength depends on the distance between the opposite charges.
By comparison, a #pi-# orbital formed by the sideways overlap of two p-orbitals locates the electrons rather further away from the bonded nuclei so their electrostatic attraction for the nuclei is a little weaker. The is why the average bond enthalpy for a C=C bond is not quite twice the bond enthalpy of a C-C bond.